Fix: suppress handled chat agent max-turn Sentry events

[ohld]

Fixes FFM-1764.

Summary

• Add chat-agent Sentry scope around the OpenAI Agents runner call so agent events carry explicit chat-agent context.
• Drop expected agents.exceptions.MaxTurnsExceeded events from the chat-agent path even when Sentry reports them with logger=null / mechanism=openai_agents.
• Add regression coverage for both scoped chat-agent events and the production webhook event shape.

Verification

• python3 -m pytest tests/test_sentry_observability.py -q
• ruff check --fix src/ tests/ && ruff format src/ tests/

Notes

• Host Python lacks the app agents.exceptions dependency and Docker is unavailable here, so tests/tgbot/test_chat_agent_runner.py could not be collected locally. CI should run it in the app dependency environment.

[ohld]

STAFF ENGINEER REVIEW: APPROVED — clean structural review. Checked Sentry suppression scope, chat-agent context lifetime, Agents SDK call sites, secret patterns, and targeted regressions. Codex review found no discrete regression. Local verification: python3 -m pytest tests/test_sentry_observability.py -q passed; tests/tgbot/test_chat_agent_runner.py could not collect on host because agents.exceptions is unavailable in this runtime.

[ohld]

✅ Approved + merged.